My U.S. Pat. No. 3,709,106 shows a linear piston operator having two axially aligned pistons connected by a bar mounting two spaced rollers for selectively engaging opposite sides of a crank arm having a collar keyed to a valve stem extending transversely through a circular notch in the piston housing, and a transverse tubular housing for the valve stem and collar cut out to conform to and enclose the notch and welded to the edges thereof. That construction provided a sealed enclosure for storing high pressure fluid in the cavity within the piston housing between the two pistons.
A disadvantage of that construction is that in an intermediate position of the valve stem between fully open and fully closed valve positions, control of the valve stem is not positive as some floating or drifting of the valve can occur because of the space between the crank arm and one of the rollers on the piston connector bar. A scotch-yoke connector having a constant sliding engagement with a single roller on the piston connector bar produces positive control of the valve in all positions, but the slot in the yoke requires substantial mass on both sides of the slot and consequent increased overall width to provide the necessary strength to absorb piston thrust. Consequently, a much larger notch in the piston housing is required to accommodate the increased width of the scotch-yoke as it oscillates between valve open and closed positions, and such notch substantially weakens the resistance of the cylinder wall to high internal pressures when high pressure fluid is stored in the cavity between the two pistons.